Syntheses, structures and properties of novel 3D lanthanide metal-organic frameworks with paddle-wheel building blocks

Three new complexes Ln(BDC)_1.5(DEF) [Ln = La(1), Ce(2), Nd(3); H₂BDC = 1,4-benzenedicarboxylic acid; DEF = N,N-diethylformamide] have been solvothermally synthesized and characterized by single-crystal X-ray diffraction. Complexes 1-3 are isostructural and crystallize in triclinic space group P over(1, ̄). They display three-dimensional frameworks and 18.7986 Å × 11.3230 Å rhombic channels along the a axis without interpenetration. Two eight-coordinated metal centers are connected with two dimonodentate BDC groups and two chelating/bridging bidentate BDC groups to give a paddle-wheel building block. The building blocks are connected through two carboxyl groups to lead to a one-dimensional infinite inorganic chain (-M-O-C-O-M-) along the [1 0 0] direction. Thermogravimetric analyses (TGA) and X-ray powder diffraction (XPRD) were determined for the three complexes. An observed intense blue luminescence with an emission band peaking at 372 nm for them may result from ligand-to-metal charge transfer (LMCT). Variable-temperature magnetic susceptibility of 2 and 3 was investigated and found deviated from the Curie-Weiss law.